This invention relates to methods for forming gratings, such as Bragg gratings, in optical fibers and, in particular, to a method for forming UV-induced gratings in optical fibers coated with UV-curable polymer without removing the polymer.
The dominant method for forming gratings in optical fiber is to induce the gratings in the fiber core by ultraviolet light shone through the fiber cladding. An optical fiber having a photosensitive glass core and a surrounding glass cladding is exposed to ultraviolet (UV) light having an intensity that varies periodically along the length of the fiber. The periodically varying intensity pattern is typically provided by applying a UV beam to an optical phase grating as described in U.S. Pat. No. 5,327,515 issued to Anderson et al. on Jul. 5, 1994, which is incorporated herein by reference. Alternatively, the varying intensity pattern can be provided by an amplitude mask or by interfering a pair of coherent UV beams as described in U.S. Pat. No. 4,725,111 issued to W. H. Glenn et al. on Feb. 16, 1988, which is incorporated herein by reference. In each of these conventional techniques, the source of UV light is typically a high intensity Excimer laser.
One of the most time-consuming steps in the conventional fabrication of a fiber grating is removal of the fiber polymer coating before exposure to the UV light. Glass optical fibers are very sensitive to contamination and mechanical damage. For protection, fibers are provided with polymer coatings immediately after they are drawn. Typically the freshly drawn fiber is passed through a bath of UV-curable pre-polymer and then moved past a UV lamp to effect on-line curing to a protective polymer. The preferred polymers are acrylates and especially urethane acrylates.
A problem arises because polymers curable by UV light are also typically opaque to the UV light used to write the gratings. Consequently it is necessary to strip the polymer coating from the fiber prior to writing the grating. Thus an initial step in conventional grating formation is stripping the polymer coating, as by soaking the fiber in hot sulfuric acid. A new coating must be applied after the grating is formed. The coating removal and reapplication steps can consume more than half the time required to form a grating.
The desirability of eliminating these rate limiting removal and reapplication steps was noted in U.S. Pat. No. No. 5,620,495 issued to Aspell et al. on Apr. 15, 1997, which is incorporated herein by reference. Aspell et al. taught that with a proper combination of low absorbing polymer, glass and low intensity UV light, gratings can be side-written into polymer coated fibers without removing the polymer. Low UV absorbing polymers, however, are typically slow to cure and require coating thicknesses and curing processes different from conventional fiber manufacture.
A refinement of the Aspell et al. approach is set forth in U.S. Pat. No. 5,773,486 issued to Chandross et al. on Jun. 30, 1998, which is incorporated herein by reference. Chandross et al. teaches that one can make special composition acrylate based polymer coatings that are both curable by UV (250-400 nm) and are sufficiently transparent to UV in the 235-260 nm range to permit gratings to be written through the coatings. The Chandross et al. polymers differ from conventional acrylate-based coatings in that the Chandross polymers are devoid of conjugated double bonds and aromatic moieties and in that they employ aliphatic or cycloaliphatic free-radical ketone photoinitiators. The Chandross polymers may also use non-aromatic thiol synergists.
It would be advantageous to provide a method for forming UV-induced gratings in optical fibers coated with more nearly conventional acrylate polymers without removing the polymer.
This invention is predicated upon applicants"" discovery that UV-induced gratings can be formed through polymer coatings that include conjugated double bonds and aromatic moieties. Coatings with low concentrations of aromatic-containing free-radical photoinitiators provide both reasonable curing speeds and sufficient transparency that gratings can be written through them. Moreover some of these aromatic-containing free-radical photoinitiators act synergistically with non-aromatic ketone photoinitiators. Advantageous aromatic-containing free-radical photoinitiator concentrations are in the range 0.01%-0.1% and preferably in the range 0.02% to 0.05%. Advantageous polymer coatings are acrylate-based coatings.